Aryl sulfone sulfonic acids are useful as leveling agents for certain nylons and as starting materials for a variety of additives especially flame-retardant additives for polymer systems. The aryl sulfone sulfonic acids have been previously prepared by several different methods as, for example, by the reaction of a phenylsulfone and chlorosulfonic acid or oleum.
Both of these methods, however, have serious drawbacks. When using chlorosulfonic acid, the primary product is the sulfonyl chloride that has to be converted to the acid by an additional hydrolysis step. This has the disadvantage, not only of requiring additional reagents and an extra reaction step, but it also requires the disposition of the co-product, hydrochloric acid. Sulfonation by oleum has, of course, the disadvantage of employing large amounts of sulfuric acid, which not only have to be separated from the desired sulfonic acid product, but also have to be disposed of in an ecologically acceptable manner. While these drawbacks of these two sulfonating reagents points to the use of liquid SO.sub.3 as the desirable reagent, the prior art of sulfonation, as exemplified by Evert Gilbert, "Sulfonation and Related Reactions", Interscience publishers, (1965), requires the use of compatible solvents.
The discovery has been made that under carefully controlled conditions, molten aryl sulfone, specifically diphenylsulfone, can be sulfonated directly in the absence of solvent by liquid SO.sub.3 with the formation of only trace amounts of sulfuric acid. The small amounts of the latter by-product can be quantitatively separated from the arylsulfone sulfonic acids by the use of stoichiometric amounts of barium hydroxide, for example. Furthermore, and surprisingly, carefully controlled conditions can also lead to diaryl sulfone disulfonic acids, again with minimum amounts of by-product sulfuric acid formation. The avoidance of solvents, few of which are chemically compatible with liquid SO.sub.3, not only results in the formation of better product but also leads to economical processes.